The present invention relates to a needle roller bearing and a supporting structure of a connecting rod using the same.
For example, in a small-sized two-cycle engine for a motor cycle, there are many cases where a needle roller bearing having no bearing ring, in which a cage and a plurality of needle rollers are assembled, is used at fitting portion between a major boss portion of a connecting rod and a crank pin and at fitting portion between a minor boss portion of a connecting rod and a piston pin. This is because the size and weight of the engine are attempted to be decreased by utilizing the advantages of this type of needle roller bearing, in that the needle roller bearing is light and has a lower sectional height and a greater load capacity.
FIG.4 shows a conventional needle roller bearing B used to support a connecting rod. The needle roller bearing B comprises a cage 13 in which a plurality of long window-shaped pockets 13a are formed at an appointed interval on the circumference, and a plurality of needle rollers 14 which are rollably accommodated and retained in the respective pockets 13a of the cage 13.
The cage 13 is an annular body in which both sides of the pockets 13a in the axial direction are annular portions 13b and both sides of each of the pockets 13a in the circumferential direction are bar portions 13c continuous to the annular portions 13b. The middle portion 13c1 of the bar portion 13c in the axial direction is recessed so that it has a smaller diameter than both-side portions 13c2 thereof in the axial direction, and the middle portion 13c1 is made continuous to the both-side portions 13c2 via inclined portions 13c3. Furthermore, the annular portions 13b protrudes toward the inner diameter side like flanges so that they have smaller diameter than both-side portions 13c2 of the bar portions 13c. Therefore, the longitudinally sectional view of the cage 13 including the bar portion 13c is made roughly M-shaped as the entirety.
The cage 13 of such a needle roller bearing B as described above is produced in such a manner that, after the roughly M-shaped basic form shown in the same drawing is obtained by lathing a steel pipe material or the like, the pockets 13a are punched out by pressing. However, conventionally, taking the performance into consideration when punching out the pockets, a connection radius R' (which is formed when securing the roughly M-shaped basic form by lathing) with the annular portion 13b, which is shown in enlargement in FIG.4(b) is set to as a small value as possible (that is, is set to R'/Dr.ltoreq.0.2 where the diameter of the needle roller 14 is Dr).
Although setting the connection radius R' to a small value improves the punching-out performance of the pockets 13a, this becomes disadvantageous in view of the contact surface pressure at a contact portion with the end surface of the needle rollers 14 because the dimension Y3 in the radius direction (and an area Y4 which is brought into contact with the end surface of the needle rollers 14) of a pocket wall surface 13a1 at the annular portion 13b side of the pocket 13a is made small. Furthermore, the connection radius R' being small becomes to be disadvantageous in view of the strength of the connection part (stress concentration) between both-side portions 13c2 of the bar portion 13c and the annular portion 13b. On the other hand, these factors regarding the durability and strength of the cage 13 can be eliminated by thickening both-side portions 13c2 of the bar portion 13c. However, this results in an increase of the weight of the cage 13 to cause the advantages of this type of needle roller bearing to be decreased.
Furthermore, since in the conventional needle roller bearing B the inside surface 13b1 of the flange-like protruding portion of the annular portion 13b of the cage 13 is formed on a plane parallel to the radius direction. Therefore, since the cutting resistance is large, the machining must be carried out with the feedrate of a cutting tool slowed down.
This type of needle roller bearing is lubricated with a lubricant oil included in a fuel-air mixture of gasoline and lubricant oil, which are sent into a crank chamber or a cylinder chamber, and is operated in an environment where contamination (worn particles of the piston, etc.) is liable to enter the bearing. Therefore, by setting the inner diameter D1' of the annular portion 13b of the cage 13 to a larger value by an appointed degree than the inscribed circle diameter D0 of needle rollers 14, thereby forming an annular clearance S' between the inner circumferential surface of the annular portion 13band the outer circumferential surface of its counterpart component (crank pin or piston pin), circulation of the lubricant oil into the bearing and discharge of contamination from the inside of the bearing are secured. Conventionally, the inner diameter D1' of the annular portion 13b of the cage 13 is set to a value having the relationship of (D0+0.15Dr).ltoreq.D1.ltoreq.(D0+0.20Dr) with respect to the inscribed circle diameter D0 of the needle rollers 14 and the diameter Dr of the needle rollers 14 (that is, equivalent to the inner diameter of the middle portion 13b1 of the bar portion 13c). However, recently, in line with the tendency of downsizing, lightening, and increasing the revolution speed and output of an engine, there are many cases where this type of needle roller bearing is used in an environment where a shortage of lubricant oil arises and contamination is liable to be accumulated. Therefore, it is requested that the lubrication performance and discharge performance of contamination are further improved.